It is known that amorphous silicon (hereinafter referred to simply as "A-Si") may be formed in a desired shape or into a large-area body. Application of amorphous silicon to various uses, for instance, electrophotographic photosensitive members, solar cells and thin-film transistors has been investigated and put to practical use. For production of deposited films of A-Si, there have been known a vacuum vapor deposition process, a thermal-induced CVD process, a sputtering process, an ion plating process, a plasma CVD process, etc., among these processes, the plasma CVD process has been highly evaluated and widely used. A variety of processes and apparatuses for carrying out such processes have been proposed, and some of them have been put to practical use.
As for the conventional A-Si deposited films, they are still not sufficient enough for their electrical and optical characteristics. And there are problems even for the plasma CVD process and also for the apparatus for carrying out the process that there are a great number of parameters due to the use of a plasma, and it is difficult to generalize such parameters. In addition, there are other problems that the use efficiency of raw material gas is low, the film deposition rate is low, etc.
In order to solve the above problems, a variety of proposals have been made. For instance, according to Japanese Patent Application Laid-Open No. 60-4047 (1985), there is proposed a method of exciting and decomposing a raw material gas for forming a deposited film by predetermined energy in a space other than a film deposition space to produce activated species, and using the activated species to form the deposited film on the surface of a substrate disposed in the film deposition space, and an apparatus for carrying out the method. It is also proposed to use microwave energy for excitation and decomposition of the film-forming raw material gas to produce the activated species.
According to the proposal mentioned above, the apparatus for carrying out the method of forming a deposited film by use of microwave energy has a construction in which a pair metallic pipe opened at both ends are connected near a trailing end portion of a rectangular waveguide on a plane perpendicular to an electric field generated, so as to penetrate through the waveguide, and a portion of a reaction vessel is extended to be contained in the pair of pipes. According to the apparatus construction, the both-end-open metallic pipes (hereinafter referred to as "microwave cutoff cavities") generally are disposed at symmetrical positions with respect to a waveguide for transmitting microwaves, or microwave waveguide, and are so designed as to prevent microwave energy from leaking to the exterior and as to have an equal microwave cutoff frequency. In addition, the apparatus is so designed that the plasma generated by the microwave energy is confined in the cavities. The microwave cutoff frequency of the cavities is generally about two times the frequency of the microwaves used.
In the apparatus proposed as above, however, the plasma generated by the microwave energy often leaks to the exterior without being confined perfectly in the microwave cutoff cavities. This phenomenon takes place in both the microwave cutoff cavity on the film deposition chamber side of the reaction vessel and the cavity on the raw material gas inlet side of the vessel. As a result, the input efficiency of the microwave energy is lowered, causing instability of the quantity of the plasma leaking, and the deposited film thus formed is uneven in film thickness and film quality becomes undesirable, leading finally to poor reproducibility of characteristics of the film. To overcome the problems, separate operations are adopted for strictly controlling the microwave power inputted, for setting the appropriate spacing between a plasma generation zone and the substrate in the film deposition chamber, and so on. In any case, the operations are complicated and troublesome, and it is difficult to obtain the desired functional deposited film steadily and stably.